a) I'm a bit confused. There is Sandscape, Cardscape, cccg, untap.in, VICE and I don't know what else.
So what is the working way (or several ways) to playtest WTactics on the web or on a PC (preferably with Linux )?
(I don't care for non-WTactics playtesting possibilties. Because WTactics has the world's best graphics in my opinion.)

b) What ways are there to playtest WTactics via a discussion-forum or chat or email?

So what is the working way (or several ways) to playtest WTactics on the web or on a PC

It would be LackeyCCG that is the easiest to setup (worked in WINE last time I checked...but not all versions), OCTGN (win only I think) or gccg (linux native). Currently no module is maintained of WT for any of those due to rules being worked on. In theory though anyone of those and several others would do the job, including "Tabletop Simulator" that seems to be released for Steam OS (= distro of linux).

Best way to playest is therefore printed paper, or using a module locally on your own computer vs yourself i.e.

What ways are there to playtest WTactics via a discussion-forum or chat or email?

I am of the other opinion that it's hard to play any CCG by forum/mail, without support software somehow so it becomes PBE-ish. I guess some kind of shared virtual desktop could work, like a common drawing site or something that allows you to put in card images and manipulate them. Search, and you will maybe find such meeting places on the net. Share if you do!

Best bet, and fastest for it's purpose, would be to meet up with a friend and just create or print cards.

- Sandscape is not neccessary because there already are LackeyCCG, OCTGN, gccg. Protest here, if I am wrong on that

- I could get started by producing/buying blank playing cards, write text on it and play against myself seeing what effects arise. I will try that when rules and cards are not perfect but somewhat playable.

- Sandscape is not neccessary because there already are LackeyCCG, OCTGN, gccg. Protest here, if I am wrong on that

It's not necessary, but neither is a card game or a computer ; )

Sandscape as a concept is cool and much needed, as it actually is a) platform independent and b) only requires a browser, so if such a tool existed it would have wide implications of what people could create, without using traditional clients.